Candidate system search and soft handoff between frequencies in a multi-carrier mobile communication system

ABSTRACT

The present invention encompasses a mobile station comprising a transmitter for transmitting outgoing signals from the mobile station and a receiver for receiving incoming signals, the receiver coupled to the transmitter and having N subreceivers, where N is an integer greater than one and each of the N subreceivers may independently be tuned to a desired frequency. The present invention also encompasses a wireless communication system comprising a plurality of base stations, where each of the base stations transmits signals on at least one of a plurality of frequencies and a plurality of mobile stations, at least one of the plurality of mobile stations comprises a transmitter for transmitting signals to at least one of the plurality of base stations and a receiver coupled to the transmitter for receiving signals from at least one of the plurality of base stations, the receiver having N subreceivers, where N is an integer greater than one and each of the N subreceivers is independently tuned to a desired frequency.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation and claims priorityto patent application Ser. No. 10/456,218 entitled “Candidate SystemSearch and Soft Handoff Between Frequencies in a Multi-Carrier MobileCommunication System” filed Jun. 6, 2003, now allowed, which is aContinuation Application claiming priority to patent application Ser.No. 09/413,648, entitled “Candidate System Search and Soft HandoffBetween Frequencies in a Multi-Carrier Mobile Communication System”filed Oct. 6, 1999, now issued as U.S. Pat. No. 6,606,485, issued Aug.12, 2003 and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to mobile communication systems.More specifically, the present invention relates to mobile communicationsystems in which mobile stations can receive and demodulate signalstransmitted on more than one frequency.

2. Description of the Related Art

FIG. 1 shows a general block diagram of an earlier mobile station 100used in a cellular telephone system, such as a code division multipleaccess (CDMA) cellular telephone system. U.S. Pat. No. 5,109,390, whichhas been assigned to the assignee of the present invention and which isincorporated herein by reference, discloses a schematic diagram of anexample of a CDMA cellular telephone system and a block diagram of amobile station used in such a system. Referring to FIG. 1, mobilestation 100 comprises an antenna 105 for communicating with basestations, a transmitter 110 for transmitting signals from the mobilestation 100, a receiver 120 for receiving signals, a searchers anddemodulators unit 130, and a duplexer 115 coupled to the antenna 105,transmitter 110, and receiver 120 for properly routing outgoing signalsfrom the transmitter 110 to antenna 105 and incoming signals fromantenna 105 to receiver 120. Duplexer 115 is a conventional duplexerthat may be a full duplexer, which allows transmitting and receivingsignals simultaneously, or a half duplexer, which allows for only one oftransmitting or receiving signals at any given time.

Receiver 120 comprises a first band pass filter (BPF) 121 coupled toduplexer 115, a mixer 122 coupled to the first BPF 121, a localoscillator (LO) 123 coupled to mixer 122, a second BPF 124 coupled tomixer 122, and a subreceiver 125 coupled to the second BPF 124.Subreceiver 125 includes a frequency translator 126, which may be eithera digital or analog frequency translator, and a third BPF 127.

Duplexer 115 routes incoming signals to the first BPF 121 which in turnsends a band passed version of the incoming signals to mixer 122. Mixer122 also receives a second input from LO 123. The output of mixer 122 issent to the second BPF 124 which sends a band passed version of itsinput signal to subreceiver 125. Frequency translator 126 receives theoutput of the second BPF 124 and shifts the received signal in thefrequency domain such that its output is centered around a desiredfrequency, namely the frequency on which signals are carried as they aretransmitted between the mobile station 100 and a base station with whichthe mobile station 100 is in communication. The third BPF 127 receivesthe output of frequency translator 126 and outputs a band passed versionof its input. The third BPF 127 has a band pass of 1.25 MHz and iscentered around the frequency on which signals are carried as they aretransmitted between the mobile station 100 and the base station withwhich the mobile station 100 is in communication. The output of thirdBPF 127 is transmitted to the searchers and demodulators unit 130. Thedemodulators in searchers and demodulators unit 130 demodulate thesignals on incoming waveforms. Thereafter, the demodulation fingers ofthe demodulators remove the codes of a communication code channel, suchas Walsh codes and pseudorandom noise (PN) codes, from the demodulatedsignals, and combine the removed codes. The searchers in the searchersand demodulators unit 130 search for the existence of a structuredwaveform, such as codes of a communication code channel, e.g., Walshcodes or PN codes. Examples of searchers and demodulators are disclosedin U.S. Pat. Nos. 5,103,459, 5,490,165, and 5,506,865, all of which havebeen assigned to the assignee of the present invention and areincorporated herein by reference. It is to be noted that in some of theabove referenced patents, a digital receiver or a digital data receivermay refer to a demodulator or a combination of a searcher anddemodulator(s). Similarly, an analog receiver may refer to what in thepresent application is referred to as receiver 120 or an equivalentthereof.

As the subreceiver is tuned to only one frequency at any given time, themobile station can be in communication only with a base stationtransmitting signals on the frequency range to which the mobile stationis tuned. This limitation with respect to the frequency to which themobile station is tuned causes the mobile station and the wirelesscommunication system within which the mobile station operates to sufferfrom several disadvantages. First, the mobile station cannot be in softhandoff between two different frequencies. Second, the mobile stationcannot monitor or search for pilots at more than one frequency at anygiven time. Third, in the idle state, the mobile station cannot monitoror search for pages at more than one frequency at any given time.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned disadvantages byusing N (where N is an integer greater than one) subreceivers in thereceiver of the mobile station. Each of the subreceivers in the mobilestation of the present invention may be independently tuned to aparticular frequency. As a result of having multiple subreceivers thatmay be independently tuned to different frequencies, the mobile stationof the present invention can simultaneously receive signals on more thanone frequency from different base stations or from different sectors ofone base station. This allows the mobile station (1) to be in (a) softhandoff between two different frequencies which are received fromdifferent base stations or (b) softer handoff between two differentfrequencies which are received from different sectors of the same basestation or (c) soft softer handoff between multiple frequencies whichare received from different base stations, where in the case of at leastone base station, multiple frequencies are received from differentsectors of the same base station, (2) to be in communication with onebase station at a first frequency while at the same time searching forand monitoring pilots at other frequencies with little or no degradationto the forward or reverse links with the base station with which it isin communication, and (3) to simultaneously search for and monitor pagesat more than one frequency in the idle state, i.e., when the mobilestation is not on a traffic channel.

In order to provide these and other benefits, the mobile stationcomprises: a transmitter for transmitting outgoing signals from themobile station; and a receiver for receiving incoming signals, thereceiver coupled to the transmitter and having N subreceivers, where Nis an integer greater than one and each of the N subreceivers mayindependently be tuned to a desired frequency.

In one embodiment, the receiver comprises three subreceivers and each ofthe three subreceivers has a frequency band that is approximately 1.25MHz wide.

In another embodiment, the receiver comprises two subreceivers. In afirst two subreceiver embodiment, the first subreceiver has a frequencyband that is two times as wide in the frequency domain as the frequencyband of the second subreceiver. In a second two subreceiver embodiment,the first subreceiver has a frequency band that is three times as widein the frequency domain as the frequency band of the second subreceiver.

The present invention also encompasses a wireless communication systemcomprising: (1) a plurality of base stations, where each of the basestations transmits signals on at least one of a plurality offrequencies; and (2) a plurality of mobile stations, where at least oneof the plurality of mobile stations comprises: (a) a transmitter fortransmitting signals to at least one of the plurality of base stations;and a receiver coupled to the transmitter for receiving signals from atleast one of the plurality of base stations, the receiver having Nsubreceivers, where N is an integer greater than one and each of the Nsubreceivers is independently tuned to a desired frequency.

In one embodiment, a first base station of the plurality of basestations transmits signals on a first frequency using a first codechannel and a second base station of the plurality of base stationstransmits signals on the first frequency using a second code channelthat is different from the first code channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of an earlier mobile station.

FIG. 2 is a general block diagram of the mobile station of the presentinvention.

FIG. 3 is a graph on the frequency domain of multiple frequencies andmultiple code channels used for transmitting signals in the wirelesscommunication system of the present invention.

FIG. 4 is a more detailed view of code channels used for transmittingsignals on one of the frequency bands in FIG. 3.

FIGS. 5-10 are tables showing examples of different combinations offrequencies, base stations, code channels, and code symbols used in thewireless communication system of the present invention.

FIG. 11 is a general block diagram of an embodiment of the multiplesubreceiver mobile station of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows a general block diagram of the mobile station of thepresently preferred embodiment of the invention. Mobile station 200comprises elements contained in mobile station 100. For each element inmobile station 200 having a corresponding element in mobile station 100,a reference number has been selected by adding 100 to the referencenumber of the corresponding element in mobile station 100. For example,the duplexer in mobile station 200 is referenced as duplexer 215, where215 is the sum of 100 and the reference number 115 for duplexer 115 inmobile station 100. As the elements in mobile station 200 havingcorresponding elements in mobile station 100 are well known to thoseskilled in the art and have been described above, those elements inmobile station 200 will not be further described herein in order toconcentrate on the inventive features of the mobile station 200 of thepresent invention. Similarly, other elements commonly used in mobilestations have been omitted from the block diagrams of mobile stations100 and 200 since such elements are well known to those skilled in theart.

As can be seen in FIGS. 1 and 2, mobile station 200 contains elements inaddition to those that have corresponding elements in mobile station100. For example, unlike receiver 120, in mobile station 100, which hasonly one subreceiver 125, receiver 220 in mobile station 200 comprises Nsubreceivers 225, where N is an integer greater than one. Each of the Nsubreceivers 225 may be independently tuned to a different frequency tosearch for, monitor, and demodulate signals sent on differentfrequencies. The use of multiple subreceivers which may be independentlytuned to different frequencies allows mobile station 200 tosimultaneously receive signals on more than one frequency from differentbase stations or different sectors of one base station.

In some embodiments, the searchers and demodulators unit 230 may containadditional demodulator BPF's (not shown) for further filtering signalsreceived from BPF's 227 in receiver 220. A demodulator would includedemodulator BPF's when the bandwidth of the BPF 227 from which thedemodulator receives signals is sufficiently wide so as to contain morethan one frequency band on which signals are commonly transmitted in awireless communication system. In a wireless communication system, inaccordance with a presently preferred embodiment of the invention, afrequency band of 1.25 MHz is commonly used to transmit signals.Therefore, when more than one 1.25 MHz band may be fitted into thefrequency band of the feeding BPF 227, then the number of 1.25 MHz bandsthat may be fitted into the frequency band of the feeding BPF 227determines the number of demodulator BPF's that should be used in ademodulator receiving signals from the feeding BPF 227. It is to benoted that the above method for determining the number of demodulatorBPF's to be used in a demodulator can be applied to systems using afrequency band different from a 1.25 MHz band for transmitting signals.For such systems, the frequency band commonly used in those systems,instead of a band of 1.25 MHz, will be used to determine the number ofdemodulator BPF's (not shown) to be used in the demodulator.

In one embodiment, N is equal to three, in which case mobile station 200comprises three subreceivers. In one embodiment having threesubreceivers, each of the subreceivers is tuned to a different frequencyand filters signals within a bandwidth of approximately 1.25 MHz. Inthis embodiment, the demodulators do not need to have any demodulatorBPF's as the bandwidth of each of BPF's 227 is equal to that offrequency bands on which signals are commonly transmitted in a wirelesscommunication system.

In a second embodiment, mobile station 200 comprises two subreceivers.In a first embodiment of a two subreceiver mobile station, onesubreceiver filters signals at a bandwidth of approximately 3.75 MHzwhile the other subreceiver filters signals at a bandwidth ofapproximately 1.25 MHz. In the above mentioned first embodiment wherethe BPF 227 of one subreceiver has a bandwidth of approximately 3.75MHz, the demodulator coupled to the 3.75 MHz BPF 227 includes threedemodulator BPF's (not shown) for subdividing the signals received fromthe 3.75 MHz BPF 227 into three separate signals each of which is in oneof three adjacent bands having a bandwidth of approximately 1.25 MHz. Inone embodiment, the band for the 1.25 MHz BPF 227 may correspond to oneof the three 1.25 MHz subbands of the 3.75 MHz BPF 227. In such anembodiment, the 3.75 MHz BPF 227 and the 1.25 MHz BPF 227 togetherinclude three demodulator BPF's (not shown) for subdividing signalsreceived from the 3.75 MHz BPF 227 and the 1.25 MHz BPF 227 into threeseparate signals each of which is in one of three adjacent bands havinga bandwidth of approximately 1.25 MHz. In a second embodiment of a twosubreceiver mobile station, one subreceiver filters signals at abandwidth of approximately 2.5 MHz while the other subreceiver filterssignals at a bandwidth of approximately 1.25 MHz. In the above mentionedsecond embodiment where the BPF 227 of one subreceiver has a bandwidthof approximately 2.5 MHz, the demodulator coupled to the 2.5 MHz BPF 227includes two demodulator BPF's (not shown) for subdividing the signalsreceived from the 2.5 MHz BPF 227 into two separate signals each ofwhich is in one of two adjacent bands having a bandwidth ofapproximately 1.25 MHz.

In one embodiment, the BPF's 227 may comprise surface acoustic wave(SAW) filters, which are well known to those skilled in the art. It isto be noted that the BPF's used in the present invention are not limitedto SAW filters, but may instead be any BPF used in the art, such asother types of analog filters (e.g., cascaded lumped elements filters,crystals) or digital filters (e.g., finite impulse response (FIR)filters).

FIG. 11 is a general block diagram of an embodiment of the multiplesubreceiver mobile station of the present invention. In mobile station1100 of FIG. 11, each of analog to digital converters (ADC's) 1125,either alone or in combination with digital signal processor 1129, maybe thought of as a subreceiver of receiver 1120. Thus, for example, theNth ADC 1125 or the Nth ADC 1125 in combination with DSP 1129 may bethought of as the Nth subreceiver. Although in the embodiment shown inFIG. 11, there is one DSP 1129 corresponding to all ADC's 1125, in analternative embodiment it is conceivable that each ADC 1125 would beassociated with a separate corresponding DSP that is not shared withother ADC's 1125.

Each ADC 1125 may be independently tuned to sample a portion of theincoming signal frequency band into digital signals. Thus, the first ADC1125 may be tuned to sample incoming signals of a frequency f1 while theNth ADC 1125 may be tuned to sample incoming signals of frequency fN,where f1 and fN are different frequencies and may be the centerfrequencies for adjacent or non-adjacent frequency bands.

In one embodiment, N is equal to three and, therefore, mobile station1100 comprises three subreceivers. In one embodiment having threesubreceivers, each of the subreceivers is tuned to a different frequencyand filters signals within a bandwidth of approximately 1.25 MHz. Mobilestation 1100 may also have the different combinations of number ofsubreceivers, subreceiver bandwidths and subreceiver frequencies (inother words, frequencies to which each subreceiver is tuned) as thosedescribed in relation to mobile station 200.

Moreover, the ADCs 1125 may be regular analog to digital converters orsigma-delta modulators. The sigma-delta modulator may be a bandpasssigma-delta modulator when the signal is an intermediate frequency (IF)signal or a low pass sigma-delta modulator when the incoming signal is abase band (i.e., an unmodulated, lower frequency) signal. U.S. Pat. No.6,005,506, issued Dec. 12, 1999 entitled “Receiver With Sigma-DeltaAnalog-To-Digital Converter” and U.S. Pat. No. 5,982,315, issued Nov. 9,1999 entitled “Multi Loop Sigma-Delta Analog-To-Digital Converter” whichhas been assigned to the assignee of the present invention and is hereinincorporated by reference provides greater detail about the sigma-deltamodulators that may be used in the present invention.

DSP 1129 receives digital signals from the ADCs 1125. Thereafter, itband pass filters each of the signals from the ADCs 1125. DSP 1129 mayalso mix the input digital signals to baseband and then use a low pass,rather than a band pass, filter for filtering the digital signals. Ifthe ADC 1125 oversamples the analog signals, in other words samples theanalog signals at a rate greater than the Nyquist rate or chip rate,then the DSP 1129 may also decimate the data from the oversampled datarate to the Nyquist rate or chip rate. Moreover, the DSP 1129 extractsthe I (in-phase) and Q (quadrature-phase) components of the signals. Inother words, the signals sent form the DSP 1129 to searchers anddemodulators 230 are the I and Q components of the input signal. U.S.Pat. No. 6,3859,069, issued May 14, 2002 entitled “A Low CurrentProgrammable Digital Filter” which has been assigned to the assignee ofthe present invention and is herein incorporated by reference providesgreater detail about the aforementioned functions of DSP 1129.

As noted above, the block diagrams of the mobile stations 100 and 200 donot show some elements commonly used in mobile stations since thoseelements are well known to those skilled in the art. Similarly, theblock diagram of mobile station 1100 also does not show some elementscommonly used in mobile stations. For example, FIGS. 1, 2, and 11 do notshow a low noise amplifier (LNA) and an automatic gain control (AGC)that are commonly used in mobile stations. Those skilled in the art,know that an LNA amplifies signals received from duplexers 115 and 215before those signals are sent to BPF's 121 and 221, respectively.Similarly, those skilled in the art know that preferably an AGC wouldcontrol the amplitude of signals output by BPF's 124 and 224 before theyare sent to subreceivers 125 and 225 (ADC 1125 in the case of mobilestation 1100). Those skilled in the art also know that the adjustment ofthe AGC is controlled based on the signal strength of the signals outputby the subreceivers.

Those skilled in the art would also realize that if the frequencytranslator 226 in mobile station 200 (or frequency translator 126 inmobile station 100) is digital, then the signals received thereby aredigitized somewhere along the signal path prior to being frequencytranslated. Similarly, those skilled in the art would realize that ifBPF 227 in mobile station 200 (or BPF 127 in mobile station 100) isanalog then the signal output thereby is converted to a digital signalfor processing by the searchers and demodulators unit 230 (or searchersand demodulators unit 130 in mobile station 100). Similarly, thoseskilled in the art would realize that the receiver (of mobile stations100 and 200) may include means for extracting the I and Q components ofsignals prior to sending those signals to the demodulators and searchersunit.

The ability to simultaneously receive signals on more than one frequencyallows the mobile station of the present invention (1) to be in (a) softhandoff between two different frequencies which are received fromdifferent base stations or (b) softer handoff between two differentfrequencies which are received from different sectors of the same basestation or (c) soft softer handoff between multiple frequencies whichare received from different base stations, where in the case of at leastone base station, multiple frequencies are received from differentsectors of the same base station, (2) to be in communication with onebase station at a first frequency while at the same time searching forand monitoring pilots at other frequencies with little or no degradationto the forward or reverse links with the base station with which it isin communication, and (3) to simultaneously search for and monitor pagesat more than one frequency in the idle state, i.e., when the mobilestation is not on a traffic channel. Additionally, the ability tosimultaneously receive signals on more than one frequency allows themobile station of the present invention to search for signals indifferent formats and technologies, such as, for example, AdvancedMobile Phone Service (AMPS), narrowband AMPS (NAMPS), and Global Systemfor Mobile (GSM).

In the search mode, while mobile station 200 (or mobile station 1100) ison a traffic channel, i.e., in continuous two-way communication with oneor more “anchor” base stations, it can tune one or more of itssubreceivers 225 (or ADC's 1125) and one or more of the base bandsearchers in searchers and demodulators unit 230 to candidatefrequencies to search for signals from other base stations or sectors ofother base stations, if sectorized antennas are used. As is known in theart, an anchor base station is a base station with which a mobilestation is in continuous two-way communication. Each of the one or moresubreceivers and base band searchers searching for signals from otherbase stations can be tuned to one of the multi-carrier frequencies ofcandidate base stations to detect the existence of the forward linksignal by measuring the forward link signal power level for therespective base station and correlating with the pilot channel, bydetecting digital control channels of analog cellular systems, or bydetecting forward link overhead channels. This search helps thedetermination of the coverage of the mobile station by different basestations and the proper timing for inter-frequency handoff. When themobile station is executing this search, one or more subreceivers andtheir corresponding base band searchers and demodulators are stillreceiving the forward traffic channel. Moreover, the mobile station 200(or mobile station 1100) continues transmitting to the anchor basestation(s) without interruption, thus ensuring the reverse link to beworking properly.

During system determination (i.e., when the mobile station isdetermining which wireless communication system it is near and whichbase station it can communicate with), the multiple subreceivers of themobile station can be used in parallel to detect forward links signalsfrom multiple base stations. Generally, parallel detection of forwardlink signals allows for quicker system determination than sequential (orserial) detection of forward link signals. In an idle state (i.e., whenthe mobile station is not in two way continuous communication with oneor more anchor base stations), the different subreceivers of the mobilestation can be tuned to receive from multiple base stations on the sameor different frequencies to improve the paging channel reliability. Themobile station searches for and monitors the paging channels frommultiple base stations using different portions of its RF front end(i.e., subreceivers) and base band demodulator. As the mobile stationand its propagation environment changes, the relative strengths of thesepaging channels may vary over time. In accordance with the presentinvention, the mobile station can monitor one or more paging channelswhile searching for the others. A portion of the RF front end of themobile station can be tuned to a new frequency or frequencies so thesearcher(s) can find strong pilot channels on a new frequency orfrequencies different from those that the mobile station is currentlymonitoring. If strong energy is detected or high correlation indicatesthe existence of a forward link signal at the new frequency orfrequencies, then the mobile station can choose to monitor the targetbase station or base stations, i.e., the base station or base stationstransmitting the pilot channels on the new frequency or frequencies.

FIG. 3 is a graph illustrating a frequency domain representation ofmultiple frequencies and multiple code channels used for transmittingsignals in the present embodiment of the wireless communication systemof the invention. In the example shown in FIG. 3, there are sixdifferent frequencies f1, f2, f3, f4, f5, and f6 used for communicatingbetween base stations and mobile stations. In FIG. 3, the frequencybands centered around each of f1, f2, f3, f4, f5, and f6 aresubstantially equal. Furthermore, the frequency bands in which f1, f2,and f3 are centered are adjacent to each other. Similarly, the frequencybands in which f4, f5, and f6 are centered are adjacent to each other.However, the frequency bands in which f3 and f4 are centered are notadjacent to each other. Base station one (BS1) and base station two(BS2) transmit signals at frequencies f1, f2, and f3 using first andsecond code channels, respectively. Similarly, base station three (BS3)and base station four (BS4) transmit signals at frequencies f4, f5, andf6 using third and fourth code channels, respectively. Therefore, as canbe seen from the above example, by using different code channels, morethan one base station may transmit signals at a given frequency. In FIG.3, signals on frequencies f1, f4, and f6 which are shown in darkeroutlines are intended for a first mobile station whereas signals carriedon frequencies f2, f3, and f5 are intended for a second mobile station.

Generally, each mobile station may receive code symbols on up to M codechannels, where M is an integer. More specifically, each demodulationfinger of the mobile station may receive code symbols on up to M codechannels. Additionally, each mobile station may receive code symbols onup to N frequencies, where, as stated above, N is an integer greaterthan one and represents the number of subreceivers within the mobilestation. N also represents the number of carriers intended for receptionby a given mobile station within the system. In the example shown inFIG. 3, M is at least six and N is equal to three.

The code symbols intended for a target mobile station may bedemultiplexed (i.e., sent as parallel substreams) on the six differentcode channels on the three frequencies which carry signals intended forthe target mobile station. Some or all of the substreams may beduplicated on the six different code channels on the three differentfrequencies which carry signals intended for the target mobile station.For example, in FIG. 3, code symbols intended for the first mobilestation may be demultiplexed or duplicated on the six code channels onfrequencies f1, f4, and f6. The code symbols may also be sent in anycombination of demultiplexing and repetition. Some combination ofdemultiplexing and repetition, an example of which is shown in FIG. 10,may be used to avoid interference and fading or to balance the load ofthe different carriers and base stations.

The mobile station multiplexes the demultiplexed symbols while itsmaximal ratio combines the repeated code symbols from differentmultipath components and different code channels on the carriers that itis receiving. Thereafter, the demultiplexed and combined code symbolsare sent to the decoder (not shown) of mobile station 200 (or mobilestation 1100).

Some code channels, such as the six code channels in FIG. 3, might notcarry any code symbols for a target mobile station. For example, some ofthe six code channels on frequencies f1, f4, and f6 might not carry anycode symbols for the first mobile station.

FIG. 4 is a more detailed representation of code channels used fortransmitting signals on one of the illustrative frequency bands in FIG.3. In FIG. 4, the illustrative carriers from BS1 and BS2 are on the samefrequency and may use the same or different code channels. Code symbolsintended for a particular mobile station can be sent by different basestations using different code channels. BS1 uses one code channel 405and one Walsh channel 410. BS2 uses three code channels 455, 465 and475, which are separated by two Walsh channels 460 and 470.

As can be seen from FIG. 6, the number of code channels on a carrierused for transmitting code symbols to a given mobile station need not beequal to the number of code channels on other carriers used fortransmitting code symbols to the same given mobile station. Moreover, agiven carrier can have a higher code symbol rate than other carriersthat are carrying code symbols to the same mobile station on the same ordifferent frequencies. This alleviates the problem of uneven load on thedifferent carriers from the same base station. The ability to send codesymbols at different rates on each carrier based on channel conditionsand the available power on each channel allows improved utilization ofchannel resources because transmitting code symbols at the same rate onall channels would force all the channels to be transmitting at the samerate as the slowest channel in the system, i.e., the channel with theleast power or requiring the highest signal to noise ratio. One way toallow for the transmission of code symbols at different code symbolrates on each carrier is by using a demultiplexing ratio between thedifferent channels that is different from a 1 to 1. The demultiplexingratio between two channels refers to the ratio of the code symboltransmission rates on the two channels. In a preferred embodiment, theresulting symbol rate on each carrier is a factor of a Walsh functionrate. An alternative approach is to demultiplex the code symbols out ofthe encoder to the carriers directly and perform the interleaving of therepeated code symbols on each channel separately.

FIGS. 5-10 are tables showing examples of different combinations offrequencies, base stations, code channels, and code symbols used in thewireless communication system of the present invention.

In FIG. 5, BS1 transmits signals, in this case code symbols S1 to S12,on frequencies f1, f2, and f3. Code symbols S1, S4, S7, and S10 aretransmitted on f1, while code symbols S2, S5, S8, and S11 aretransmitted on f2, and code symbols S3, S6, S9, and S12 are transmittedon f3. Thus, BS1 transmits multiplexed code symbols S1 to S12 onconsecutive frequencies f1, f2, and f3.

In the example of FIG. 5, the code symbol flow on the forward linkremains the same before, during, and after the search. Maintaining thesame code symbol flow on the forward link results in an effectivelyincreased error-correcting coding rate. Missing code symbols or energycaused by the search are known to the receiver and are treated aserasures. Using methods and decision trees well known to those skilledin the art, depending on the resulting bit energy to noise density ratio(Eb/Nt), the fading scenario, and the awareness of the anchor basestation about the search, the forward link traffic channel power for themobile station may be increased or otherwise adjusted to ensure that theforward link quality is sufficient.

In another embodiment, the forward link code symbols intended for themobile station are sent only on the remaining carriers, i.e., thecarriers that the mobile station is still demodulating in the searchmode.

It is to be noted that the above problem of missing code symbols existswhen the subreceivers of the mobile station do not have a combinedbandwidth that is sufficient for both receiving signals on frequenciesf1, f2, and f3 and the search frequency. In some embodiments, the codesymbols transmitted on frequencies f1, f2, and f3 may be receivedwithout missing code symbols while searching for signals at anotherfrequency. For example, in the embodiment of the present invention whereone subreceiver has a 3.75 MHz BPF and another subreceiver has a 1.25MHz BPF, the subreceiver with the 3.75 MHz BPF can receive the codesymbols transmitted on frequencies f1, f2, and f3 while the subreceiverwith the 1.25 MHz BPF searches for signals on another frequency. In sucha case, code symbols on frequencies f1, f2, and f3 would not be missedat the mobile station due to the searching by the subreceiver with the1.25 MHz BPF. Alternatively, one 1.25 MHz subband of the 3.75 MHz BPFmay be used for searching while its remaining two 1.25 MHz subbands andthe 1.25 MHz BPF are used for receiving code symbols on frequencies f1,f2, and f3.

In FIG. 6, BS1 transmits code symbols on f1 and f2 using code channelsC1 and C2. BS1 transmits code symbols S1, S4, S7, and S10 on f1 usingcode channel C1. BS1 also transmits code symbols S2, S5, S8, and S11 onf1 using code channel C2. BS1 also transmits code symbols S3, S6, S9,and S12 on f2 using code channel C1. Code channels C1 and C2 may beWalsh code channels.

In FIG. 6, BS1 uses only two frequencies f1 and f2, rather than threefrequencies f1, f2, and f3 as in the example shown in FIG. 5, fortransmitting code symbols S1 to S12. However, BS1, in the example ofFIG. 6, uses two, rather than only one Walsh code channel as in theexample shown in FIG. 5, on frequency f1. The use of more Walsh codechannels allows BS1, in the example of FIG. 6, to transmit code symbolsat the same rate as that in the example of FIG. 5. Thus, the code symbolthroughput may be maintained by using a higher symbol rate per carrier,such as by use of more Walsh code channels per carrier. The code symbolthroughput may also be reduced which would result in a highererror-correcting code rate and less redundancy.

A code symbol distribution such as that of the example shown in FIG. 6,where only two frequencies are used for transmitting code symbols to themobile station on the forward link, allows a mobile station which cansimultaneously receive signals on only three frequencies to receive codesymbols on the forward link without missing code symbols due tosearching on a third frequency. For example, while subreceivers tuned tofrequencies f1 and f2 receive code symbols on the forward link, thesubreceiver tuned to frequency f3 may be used to search for signals fromother base stations or different sectors of the same base station ifsectorized antennas are used.

A code symbol distribution such as that shown in FIG. 6 may also be usedwhen the load on the frequency f3 from BS1 is relatively high and thaton frequency f1 is relatively low.

In FIG. 7, BS1 transmits code symbols S1, S4, S7, and S10 on f1, codesymbols S2, S5, S8, and S11 on f2, and code symbols S3, S6, S9, and S12on f3 while BS2 transmits code symbols S2, S5, S8, and S11 on f1. In theexample of FIG. 7, some of the multiplexed code symbols, specificallycode symbols S2, S5, S8, and S11, are sent by both BS1 and BS2 on f2 andf1, respectively, for increased reliability at the cost of redundancy,as BS2 does not send information additional to that sent by BS1. Suchredundancy for improved reliability may be particularly appropriate in asituation where there is soft handoff between BS1 and BS2 across twodifferent frequencies f2 and f1.

In FIG. 8, BS1 transmits code symbols S1, S4, S7, and S10 on f1, codesymbols S2, S5, S8, and S11 on f2, and code symbols S3, S6, S9, and S12on f3 while BS2 transmits code symbols S1, S4, S7, and S10 on f1, codesymbols S2, S5, S8, and S11 on f2, and code symbols S3, S6, S9, and S12on f3. In the example of FIG. 8, there is total redundancy in the codesymbols transmitted by BS1 and BS2 as the code symbols transmitted byBS1 on f1, f2, and f3 are also transmitted by BS2 on the samefrequencies. This increased redundancy results in increased reliability.

In FIG. 9, BS1 transmits code symbols S1, S7, S13, and S19 on f1, codesymbols S2, S8, S14, and S20 on f2, and code symbols S3, S9, S15, andS21 on f3 while BS3 transmits code symbols S4, S10, S16, and S22 on f1,code symbols S5, S11, S17, and S23 on f2, and code symbols S6, S12, S18,and S24 on f3. In the example of FIG. 9, there is no redundancy and theincreased reliability that results therefrom as the code symbols aredemultiplexed onto frequencies from different base stations forincreased throughput.

In FIG. 10, BS1 transmits code symbols S1, S4, S7, and S10 on f1, BS2transmits code symbols S1, S4, S7, and S10 on f1, BS3 transmits codesymbols S2, S5, S8, and S11 on f4 and code symbols S3, S6, S9, and S12on f6, and BS4 transmits code symbols S2, S5, S8, and S11 on f4 and codesymbols S3, S6, S9, and S12 on f6. In the example of FIG. 10, codesymbols are demultiplexed onto different frequencies, but more than onebase station transmits the same code symbols on each frequency in orderto avoid interference and fading. In the example of FIG. 10, there issoft handoff between BS1 and BS2 on frequency f1. Similarly, there issoft handoff between BS3 and BS4 on frequencies f4 and f6.

While the present invention has been particularly described with respectto the illustrated embodiments, it will be appreciated that variousalterations, modifications and adaptations may be made based on thepresent disclosure, and are intended to be within the scope of thepresent invention. While the invention has been described in connectionwith what are presently considered to be the most practical andpreferred embodiments, it is to be understood that the present inventionis not limited to the disclosed embodiments but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. A mobile station comprising: a transmitter for transmitting outgoingsignals from the mobile station; a receiver for receiving incomingsignals, said receiver coupled to said transmitter and having Nsubreceivers, wherein N is an integer greater than one and each of saidN subreceivers may independently be tuned to a desired frequency, eachsubreceiver having a band pass filter, wherein at least one of the bandpass filters has a frequency bandwidth at least twice as wide as anotherone of the band pass filters; and a demodulator coupled to an output ofat least two of the N subreceivers for demodulating output signals fromeach of the at least two of the N subreceivers.
 2. The mobile station ofclaim 1, wherein said N subreceivers are tuned to frequency bands havingequal frequency widths.
 3. The mobile station of claim 2, wherein saidreceiver comprises three subreceivers and each of the frequency bands is1.25 MHz wide.
 4. The mobile station of claim 1, wherein a firstsubreceiver and a second subreceiver of said N subreceivers have a firstfrequency band and a second frequency band, respectively, and said firstfrequency band is two times as wide in the frequency domain as saidsecond frequency band.
 5. The mobile station of claim 1, wherein a firstsubreceiver and a second subreceiver of said N subreceivers have a firstfrequency band and a second frequency band, respectively, and said firstfrequency band is three times as wide in the frequency domain as saidsecond frequency band.
 6. The mobile station of claim 1, wherein each ofsaid N subreceivers comprises a surface acoustic wave filter.
 7. Themobile station of claim 1, wherein each of said N subreceivers comprisesan analog to digital converter.
 8. The mobile station of claim 7,wherein said N subreceivers further comprise a digital signal processor.9. The mobile station of claim 1, wherein each of said N subreceiverscomprises a sigma-delta modulator.
 10. The mobile station of claim 9,wherein said N subreceivers further comprise a digital signal processor.11. The mobile station of claim 1 further comprising: a duplexer coupledto said receiver and transmitter; an antenna coupled to said duplexer; asearcher coupled to said receiver, the demodulator being further coupledto said searcher.
 12. A wireless communication system comprising: aplurality of base stations, wherein each of said base stations transmitssignals on at least one of a plurality of frequencies; a plurality ofmobile stations, at least one of said plurality of mobile stationscomprises: a transmitter for transmitting signals to at least one ofsaid plurality of base stations; a receiver coupled to said transmitterfor receiving signals from at least one of said plurality of basestations, said receiver having N subreceivers, wherein N is an integergreater than one and each of said N subreceivers is independently tunedto a desired frequency, each subreceiver having a band pass filter,wherein at least one of the band pass filters has a frequency bandwidthat least twice as wide as another one of the band pass filters; and ademodulator coupled to an output of at least two of the N subreceiversfor demodulating output signals from each of the at least two of the Nsubreceivers.
 13. The wireless communication system of claim 12, whereinsaid N subreceivers are tuned to frequency bands having equal frequencywidths.
 14. The wireless communication system of claim 13, wherein saidreceiver comprises three subreceivers and each of the frequency bands is1.25 MHz wide.
 15. The wireless communication system of claim 12,wherein a first subreceiver and a second subreceiver of said Nsubreceivers have a first frequency band and a second frequency band,respectively, and said first frequency band is two times as wide in thefrequency domain as said second frequency band.
 16. The wirelesscommunication system of claim 12, wherein a first subreceiver and asecond subreceiver of said N subreceivers have a first frequency bandand a second frequency band, respectively, and said first frequency bandis three times as wide in the frequency domain as said second frequencyband.
 17. The wireless communication system of claim 12, wherein each ofsaid N subreceivers comprises a surface acoustic wave filter.
 18. Thewireless communication system of claim 12, wherein each of said Nsubreceivers comprises an analog to digital converter.
 19. The wirelesscommunication system of claim 18, wherein said N subreceivers furthercomprise a digital signal processor.
 20. The wireless communicationsystem of claim 12, wherein each of said N subreceivers comprises asigma-delta modulator.
 21. The wireless communication system of claim20, wherein said N subreceivers further comprise a digital signalprocessor.
 22. The wireless communication system of claim 12, whereinsaid at least one mobile station further comprises: a duplexer coupledto said receiver and said transmitter; an antenna coupled to saidduplexer; a searcher coupled to said receiver, the demodulator beingfurther coupled to said searcher.
 23. The wireless communication systemof claim 12, wherein a first base station of said plurality of basestations transmits signals on a first frequency using a first codechannel and a second base station of said plurality of base stationstransmits signals on said first frequency using a second code channel,said first and second code channels being different code channels. 24.In a wireless communication system, a method of communicating between amobile station and at least one base station, said method comprising:receiving and demodulating first signals at a first frequency from afirst base station; while receiving and demodulating the first signals,receiving and demodulating second signals from one of a second basestation and a second sector of the first base station, wherein thesecond signals are at a second frequency that is different from thefirst frequency, and wherein the demodulating of each of the first andsecond signals is performed at a demodulator coupled to an output ofeach of at least two of N subreceivers, each subreceiver having a bandpass filter, wherein at least one of the band pass filters has afrequency bandwidth at least twice as wide as another one of the bandpass filters; searching for and monitoring pages at the first frequency;and while searching for and monitoring pages at the first frequency,searching for and monitoring pages at the second, frequency.
 25. In awireless communication system, a method of communicating between amobile station and at least one base station, said method comprising:receiving and demodulating first signals at a first frequency from afirst base station; while receiving and demodulating the first signals,receiving and demodulating second signals from one of a second basestation and a second sector of the first base station, wherein thesecond signals are at a second frequency that is different from thefirst frequency, and wherein the demodulating of each of the first andsecond signals is performed at a demodulator coupled to an output ofeach of at least two of N subreceivers, each subreceiver having a bandpass filter, wherein at least one of the band pass filters has afrequency bandwidth at least twice as wide as another one of the bandpass filters; searching the first signals for existence of a structuredwaveform within the first signals, wherein demodulating the firstsignals comprises removing structured waveforms from the first signals;transmitting first outgoing signals to the first base station; and whilesearching and demodulating the first signals and transmitting the firstoutgoing signals, searching for pilots at the second frequency.
 26. Themethod of claim 25, further comprising: establishing a communicationlink with the second base station at the second frequency wherein themobile station is in soft handoff between the first and secondfrequencies with the first base station and the second base station. 27.The method of claim 25, further comprising: establishing a communicationlink with the second sector of the first base station at the secondfrequency wherein the mobile station is in softer handoff between thefirst and second frequencies with the first base station and the secondsector of the first base station.
 28. The method of claim 25, furthercomprising: receiving third signals at a third frequency from the secondsector of the first base station; establishing a communication link withthe second base station at the second frequency; establishing acommunication link with the second sector of the first base station;wherein the mobile station is in soft softer handoff between the first,the second, and the third frequencies with the first base station, thesecond base station and the second sector of the first base station. 29.A method for communicating between a mobile station and at least onebase station comprising: receiving and demodulating first signals at afirst frequency from a first base station; receiving and demodulatingsecond signals from one of a second base station and a second sector ofthe first base station while receiving said first signals, said secondsignals being at a second frequency that is different from the firstfrequency, and wherein the demodulating of each of the first and secondsignals is performed at a demodulator coupled to an output of each of atleast two of N subreceivers, each subreceiver having a band pass filter,wherein at least one of the band pass filters has a frequency bandwidthat least twice as wide as another one of the band pass filters; andestablishing a first communication link with the second base station atthe second frequency, wherein the mobile station is in at least one ofsoft handoff and softer handoff between the first and second frequencieswith the first base station and the second sector of the first basestation.
 30. The method of claim 29, further comprising: receiving thirdsignals at a third frequency from the second sector of the first basestation; establishing a second communication link with the second basestation at the second frequency; and establishing a third communicationlink with the second sector of the first base station; and wherein themobile station is in soft softer handoff between the first, the second,and the third frequencies with the first base station, the second basestation and the second sector of the first base station.
 31. A mobilestation comprising: transmitter means for transmitting outgoing signalsfrom the mobile station; receiver means for receiving incoming signals,said receiver means coupled to said transmitter means and having Nsubreceiver means, wherein N is an integer greater than one and each ofsaid N subreceivers may independently be tuned to a desired frequency,each subreceiver having a band pass filter, wherein at least one of theband pass filters has a frequency bandwidth at least twice as wide asanother one of the band pass filters; and demodulator means coupled toan output of at least two of the N subreceivers for demodulating outputsignals from each of the at least two of the N subreceivers.